The electronic structure of aromatic molecules. Non-empirical calculations on indole, benzofuran, benzothiophen, and related hydrocarbons

Abstract

Ab initio calculations in a gaussian orbital basis (LCGO) are reported for naphthalene, indene, styrene, indole, benzofuran, and benzothiophen. A bond energy analysis on small non-aromatic molecules yields energies for the classical Kekulé structures; the resonance energies derived for the real molecules, in close agreement with figures based upon thermochemical data, are: naphthalene 357, indene 225, styrene 208, indole 308, benzofuran 232, benzothiophen 183, benzene 212 kJ mol^–1. The calculated dipole moments are in reasonable agreement with experiment, and are separated into σ- and π-components. The high values of the π-moments, and the degree of scatter of the π-electrons into isolated pairs are related to aromatic character. Although the molecules are not isoelectronic, extensive correlation of the orbitals in the series can be achieved, and the He^I photoelectron spectra are interpreted in terms of the main groups of orbital energies.